C code generator

[jklmnn]

While the generated SPARK code is already architecture independent there are still platforms that don't have an Ada compiler at all. To increase the portability of RecordFlux a C code generator can be added.

The C code generator would consist of two major parts:

• A C equivalent to the ada.py package for generating the message parser and generator code
• A C backend for the Intermediate representation for sessions #1204

Additionally the rflx executable needs to be adapted to allow the selection of the desired output language. For that I propose the following parameter:

--target-language=<language>

The default value for the language would be spark. This pattern allows to introduce further languages in the future without needing to add new parameters.